The present invention relates to a shift control system for an automatic transmission.
An example of a shift control system is included in a hydraulic circuit of an A340E type automatic transmission described in a Service Manual of TOYOTA SUPRA issued in February 1989. The A340E type automatic transmission includes an auxiliary gear train coupled with a 3-speed main gear train. The auxiliary gear train includes a planetary gear set, a one-way clutch, a direct clutch and an OD brake, and is constructed such that the planetary gear set is locked to rotate as a unit when the direct clutch is engaged, and the planetary gear set is in an accelerating state or an over-drive state when the OD brake is engaged. Upon shifting from the 4th speed to the 3rd speed, it is necessary to release the OD brake and engage the direct clutch. Thus, a hydraulic pressure supply to the direct clutch and the OD brake is controlled by a 3-4 shift valve. In this case, the direct clutch should be engaged after the OD brake is completely released. A torque can be transmitted by the oneway clutch in the event that the OD brake becomes disengaged. The direct clutch engaged in this state assures smooth shifting. If the direct clutch is engaged before the OD brake is completely disengaged, there arises an interlock state that two friction elements are simultaneously engaged, causing inconvenience such as a great shift shock. For preventing such inconvenience, an accumulator is disposed to a hydraulic passage through which the direct clutch is subject to a hydraulic pressure. The hydraulic pressure actuating the direct clutch is kept low while a piston of the accumulator strokes so that the direct clutch cannot be engaged. When the stroke of the piston is completed after a predetermined period of time, the hydraulic pressure of the direct clutch is increased. In this manner, since the direct clutch is not engaged during the stroke of the piston, the hydraulic pressure of the OD brake is discharged, thus disengaging the brake. Specifically, the accumulator gives a time lag on engagement of the direct clutch.
With the above-mentioned shift control system, however, the accumulator always gives the time lag on engagement of the direct clutch, so that shifting is appropriately performed during a normal run of a vehicle. However, a shift responsibility is not satisfactory in the event that an engine brake is required. Specifically, the time lag is not regarded as considerable in the event of 4-3 shifting with an increase of depression of an accelerator pedal, while the time lag is regarded as considerable in the event of selecting to 3 range because a driver will expect an immediate occurrence of an engine brake effect.
Therefore, an object of the present invention is to provide a shift control system for an automatic transmission which gives a quick shift responsibility on engine brake.